This invention relates to infrared radiation sources, and more particularly, to a new and improved type of infrared generator.
The transfer of heat at high temperatures is much more effective and efficient by radiation than by conduction and convection. Transfer of heat by radiation takes place by the fourth power of the differences in temperature, and is expressed by the familiar Stefan-Boltzman equation: EQU Q=AK(T.sub.1.sup.4 -T.sub.2.sup.4)
where:
Q=heat transferred in thermal units/hour; PA0 T.sub.1 =high level temperature; PA0 T.sub.2 =low level temperature; PA0 A=area of heat transfer surface; and PA0 K=an appropriate coefficient. PA0 t.sub.1 =high level temperature; PA0 t.sub.2 =low level temperature; and PA0 C=an appropriate coefficient.
Transfer of heat by conduction and convection, on the other hand, takes place only by the first power difference of the temperature, and is expressed by the equation: EQU Q=AC(t.sub.1 -t.sub.2)
where:
Electromagnetic radiation generates heat in any absorbing object lying in its path by causing vibrations or rotations within the atomic structure of the object. Infrared radiation is commonly thought of when transfer of heat by radiation is considered. However, objects placed at the focal point of a lens in a high-density beam of visible light or close to the source of a high-intensity microwave beam, also become heated by a similar process.
The frequency band of infrared radiation ranges from approximately 1 million to 500 million megahertz. This lies between the higher frequency region of visible light and the lower frequency region of microwaves. In terms of wavelength, the infrared spectrum lies between 0.72 and approximately 1,000 microns, that is, between the borders of visible light at the shorter wavelength end and of microwaves at the longer wavelength end of the infrared spectrum. See FIG. 1. Infrared radiation can be optically focused and directed by lenses or mirrors, or dispersed by prisms. At the same time it can be transmitted like radio or radar waves through materials which are opaque to visible light. Consequently, infrared radiation exhibits some of the characteristics of both visible light and of radar and radio waves, and in certain applications has outstanding advantages over both these forms of electromagnetic radiation, thereby accounting for its widespread popularity in heat radiation applications. Four quite natural, though purely arbitrary divisions of the infrared spectrum are commonly used. These are: (i) the "near-infrared" region from approximately 0.7 microns to 3 microns; (ii) the "middle-infrared" region from approximately 3 to 6 microns; (iii) the "far-infrared" region from approximately 6 to 15 microns; and (iv) the "extreme-infrared" region from approximately 15 to 1,000 microns.
The spectrum of every thermal source will always contain infrared radiation, and for all heater temperatures now attainable, most of the radiation actually falls in the infrared region. Thus, any modern thermal radiator is essentially an infrared source. An example of this would be a simple, closely wound spiral of nichrome wire, the wire being heated to an approximate range of 1200.degree. to 1500.degree. K. by passage of electric current. A disadvantage, however, of the nichrome spiral is that optical images of the source are not uniform. This disadvantage may be overcome by surrounding the coil with a uniform ceramic tube, which in turn is heated to approximately 1400.degree. K. by the nichrome coil, an arrangement that combines the desirable properties of nichrome with the desirable optical properties of the uniform source. The ceramic sleeve over the nichrome heater is done so that a thermal mass is provided to optically diffuse the infrared radiation being emitted by the nichrome. The ceramic sleeve is absorbing the nichrome's infrared radiation and reradiating the infrared radiation in a diffuse uniform output. The infrared radiation from the ceramic sleeve is never greater than the infrared radiation from the nichrome heater and has the same spectral output as the heater. The disadvantage of this type arrangement is that the infrared radiation is not focused within any of the infrared regions. Therefore, it does not follow that every incandescent body makes a good infrared source. In the prior art there are several infrared generators which are known and which are used for radiating, concentrating, and transporting energy. Included among these are: the Nernst glower, the globar, the Welsbach mantle, and the infrared incandescent lamp.
The Nernst glower consists of a cylindrical rod or tube of refractory oxides, e.g., zirconium oxide, yttrium oxide, etc. A flexible platinum conductor is attached to each end of the rod and electrical current applied to the conductors. Since the Nernst glower's operating temperature is about 2,000.degree. K., starting temperature is usually achieved with the aid of an auxiliary heater such as a coil of platinum wire wound on a ceramic form which is placed in close proximity to the glower and backed by a reflector. Once the glower is operating satisfactorily, the auxiliary heater is turned off. The rod reaches incandescence directly in the air and then becomes a powerful source of infrared radiation up to 14 microns with peak infrared radiation output at about 2 microns.
The globar is a rod of bonded silicon carbide capped with metallic caps which serve as electrodes for the conduction of current through the globar from a power source. The globar conducts readily at normal temperatures and so does not need to be heated on starting. The passage of current causes the globar to heat yielding radiation at a temperature above 1,000.degree. C.
The Welsbach mantle or gas mantle consists essentially of a light cloth jacket prepared largely with thorium oxide and heated by a hot gas or oil vapor. The radiation of this source in the range from 0.7 to 8 microns is small, but it increases with wavelength, and beyond.
The Nernst glower emits the most energy per unit area from 2 to 14 microns. The mantle surpasses the Nernst glower at 14 microns and the globar surpasses the Nernst glower at 15.5 microns. The mantle is superior to both the globar and Nernst glower from 14 to 25 microns. Beyond 25 microns the Nernst glower is approximately equivalent to a 900.degree. C. blackbody, and the globar and mantle are 1.4 times better than the Nernst glower at these longer wavelengths.
Incandescent lamps with carbon, tungsten, and other heater elements serve also as sources of infrared radiation. The tungsten filament lamps radiate 50% of their power in the wavelength range from 0.75 to 1.4 microns, and 33% in the wavelength range beyond 1.4 microns. The carbon filament lamps radiate primarily in the spectral region from 1.5 to 2.5 microns. For equal temperatures, the relative total power in the infrared spectral region is higher for the carbon filament than for the tungsten filament lamps. However, bulbs of the carbon lamps tend to blacken because of intensive sputtering of the filaments.
The above described enfrared generators emit infrared radiation maximums corresponding to black body radiation, and emit primarily in the near, middle and far infrared regions. Emissions above 40 microns are limited.